Athermalized multi-path interference filter

ABSTRACT

A multi-path interference filter. The multi-path interference filter includes a first port waveguide, a second port waveguide, and an optical structure connecting the first port waveguide and the second port waveguide. The optical structure has a first optical path from the first port waveguide to the second port waveguide, and a second optical path, different from the first optical path, from the first port waveguide to the second port waveguide. The first optical path has a portion, having a first length, within hydrogenated amorphous silicon. The second optical path has a portion, having a second length, within crystalline silicon, and the second optical path has either no portion within hydrogenated amorphous silicon, or a portion, having a third length, within hydrogenated amorphous silicon, the third length being less than the first length.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a continuation of U.S. patent applicationSer. No. 16/816,142, filed Mar. 11, 2020, entitled “ATHERMALIZEDMULTI-PATH INTERFERENCE FILTER”, which is a continuation of U.S. patentapplication Ser. No. 16/036,866, filed Jul. 16, 2018, entitled“ATHERMALIZED MULTI-PATH INTERFERENCE FILTER”, which claims priority toand the benefit of U.S. Provisional Application No. 62/533,545, filedJul. 17, 2017, entitled “A-SI:H FOR ATHERMAL WDM”, the entire contentsof all documents identified in this paragraph are hereby incorporatedherein by reference as if fully set forth herein.

FIELD

One or more aspects of embodiments according to the present inventionrelate to arrayed waveguide gratings, and more particularly to animproved arrayed waveguide grating design.

BACKGROUND

Arrayed waveguide gratings (AWGs) may be used in various applications,to route light according to its wavelength. Rectangular AWGs may havevarious favorable characteristics, including compactness, but the numberof channels and the channel spacing achievable with such devices may belimited by constraints on the transverse separation between waveguidesof the array. Moreover, the behavior of an AWG, or of another multi-pathinterference filter, may be temperature-dependent.

Thus, there is a need for an improved multi-path interference filterdesign.

SUMMARY

According to an embodiment of the present disclosure there is providedan arrayed waveguide grating, including: a first star coupler; a secondstar coupler; an array of waveguides connecting the first star couplerand the second star coupler; one or more first port waveguides connectedto the first star coupler; and one or more second port waveguidesconnected to the second star coupler, wherein: a first optical path,from a first waveguide of the first port waveguides, through a firstwaveguide of the array of waveguides, to a first waveguide of the secondport waveguides, includes a portion, having a first length, withinhydrogenated amorphous silicon, the remainder of the first optical pathis within crystalline silicon, a second optical path, from the firstwaveguide of the first port waveguides, through a second waveguide ofthe array of waveguides, to the first waveguide of the second portwaveguides, includes a portion, having a second length, withinhydrogenated amorphous silicon, the remainder of the second optical pathis within crystalline silicon, and the second length is different fromthe first length.

In one embodiment, a rate of change, with temperature, of a centerwavelength of a channel of the arrayed waveguide grating is less than 70pm/° C.

In one embodiment, the first waveguide of the array of waveguidesincludes a first portion, having a length equal to the first length,composed of hydrogenated amorphous silicon, the remainder of the firstwaveguide of the array of waveguides is composed of crystalline silicon,the second waveguide of the array of waveguides includes a portion,having a length equal to the second length, composed of hydrogenatedamorphous silicon, and the remainder of the second waveguide of thearray of waveguides is composed of crystalline silicon.

In one embodiment, an interface between the first portion of the firstwaveguide and a portion of the remainder of the first waveguide is asubstantially planar surface having a surface normal, an angle betweenthe surface normal and a longitudinal direction of the first portionbeing greater than 0.1 degrees.

In one embodiment, the angle between the surface normal and thelongitudinal direction of the first portion is less than 30 degrees.

In one embodiment, the first star coupler includes a free propagationregion including an area composed of hydrogenated amorphous silicon, theremainder of the free propagation region of the first star coupler beingcomposed of crystalline silicon, the area including a wedge-shapedportion.

According to an embodiment of the present disclosure there is provided amulti-path interference filter, including: a first port waveguide; asecond port waveguide; and an optical structure connecting the firstport waveguide and the second port waveguide, the optical structurehaving: a first optical path from the first port waveguide to the secondport waveguide, and a second optical path, different from the firstoptical path, from the first port waveguide to the second portwaveguide, the first optical path having a portion, having a firstlength, within hydrogenated amorphous silicon, the second optical pathhaving a portion, having a second length, within crystalline silicon,and the second optical path having either no portion within hydrogenatedamorphous silicon, or a portion, having a third length, withinhydrogenated amorphous silicon, the third length being less than thefirst length.

In one embodiment, an optical path delay difference between the firstoptical path and the second optical path has a rate of change, withtemperature, of less than 2e-5 radians/° C.

In one embodiment, the optical structure comprises a Mach-Zehnderinterferometer having: a first coupler, a second coupler, a firstwaveguide connecting the first coupler and the second coupler, and asecond waveguide connecting the first coupler and the second coupler,wherein: a portion of the first optical path is within the firstwaveguide, and a portion of the second optical path is within the secondwaveguide.

In one embodiment, the optical structure comprises a generalizedMach-Zehnder interferometer having: a first coupler, a second coupler, afirst waveguide connecting the first coupler and the second coupler, asecond waveguide connecting the first coupler and the second coupler,and a third waveguide connecting the first coupler and the secondcoupler, wherein: a portion of the first optical path is within thefirst waveguide, and a portion of the second optical path is within thesecond waveguide.

In one embodiment, the optical structure includes two concatenatedMach-Zehnder interferometers.

In one embodiment, the optical structure includes an echelle grating.

In one embodiment, an optical path delay difference between the firstoptical path and the second optical path exhibits a maximum change, overa temperature range extending from 20° C. to 70° C., of less than 2e-3radians.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the present invention will beappreciated and understood with reference to the specification, claims,and appended drawings wherein:

FIG. 1A is a plan view of a rectangular arrayed waveguide grating,according to an embodiment of the present invention;

FIG. 1B is a graph of the free spectral range as a function of theincremental delay length, according to an embodiment of the presentinvention;

FIG. 1C is a schematic drawing of a rectangular arrayed waveguidegrating, according to an embodiment of the present invention;

FIG. 2A is a schematic drawing of a rectangular arrayed waveguidegrating, according to an embodiment of the present invention;

FIG. 2B is a schematic drawing of a T-shaped arrayed waveguide grating,according to an embodiment of the present invention;

FIG. 3 is a plan view of a T-shaped arrayed waveguide grating, accordingto an embodiment of the present invention;

FIG. 4A is a cross section of a rib waveguide, according to anembodiment of the present invention;

FIG. 4B is a cross section of a strip waveguide, according to anembodiment of the present invention;

FIG. 4C is a cross section of a portion of an array of rib waveguides ona shared slab, according to an embodiment of the present invention;

FIG. 4D is a portion of a cross section of a rib to strip converter,according to an embodiment of the present invention;

FIG. 5 is a schematic drawing of a T-shaped arrayed waveguide grating,according to an embodiment of the present invention;

FIG. 6A is a schematic drawing of a tunable T-shaped arrayed waveguidegrating, according to an embodiment of the present invention;

FIG. 6B is a schematic drawing of a tunable T-shaped arrayed waveguidegrating, according to an embodiment of the present invention;

FIG. 7 is a schematic drawing of an athermal arrayed waveguide grating,according to an embodiment of the present invention;

FIG. 8 is a drawing of an athermal arrayed waveguide grating, accordingto an embodiment of the present invention;

FIG. 9 is a drawing of an athermal arrayed waveguide grating, accordingto an embodiment of the present invention;

FIG. 10 is a drawing of an athermal arrayed waveguide grating, accordingto an embodiment of the present invention;

FIG. 11A is a drawing of an athermal arrayed waveguide grating,according to an embodiment of the present invention;

FIG. 11B is a drawing of an athermal arrayed waveguide grating,according to an embodiment of the present invention;

FIG. 12A is a schematic drawing of an athermal Mach-Zehnderinterferometer, according to an embodiment of the present invention;

FIG. 12B is a schematic drawing of an athermal lattice filter, accordingto an embodiment of the present invention;

FIG. 12C is a schematic drawing of an athermal generalized Mach-Zehnderinterferometer, according to an embodiment of the present invention;

FIG. 13A is a drawing of an athermal echelle grating, according to anembodiment of the present invention; and

FIG. 13B is an enlarged view of portion 13B of FIG. 13A, according to anembodiment of the present invention.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of aT-shaped arrayed waveguide grating provided in accordance with thepresent invention and is not intended to represent the only forms inwhich the present invention may be constructed or utilized. Thedescription sets forth the features of the present invention inconnection with the illustrated embodiments. It is to be understood,however, that the same or equivalent functions and structures may beaccomplished by different embodiments that are also intended to beencompassed within the spirit and scope of the invention. As denotedelsewhere herein, like element numbers are intended to indicate likeelements or features.

Referring to FIG. 1A, in some embodiments a rectangular arrayedwaveguide grating (AWG) may be used to direct light from an inputwaveguide 110 to one of a plurality of output waveguides 120 accordingto the wavelength of the light. Light from the input waveguide 110illuminates, at a first star coupler 130, each waveguide 140 of an array145 of waveguides 140, each of which has a different length. At a secondstar coupler 150, the light exiting the waveguides 140 may interfereconstructively at one of the output waveguides 120.

The output waveguide at which the constructive interference occursdepends on the wavelength of the light; accordingly, a wavelength may beassociated with each output waveguide 120. The wavelength (or frequency)difference between the wavelengths corresponding to two adjacent outputwaveguides is referred to herein as the “channel spacing”. The AWG maybe a reciprocal device, e.g., for light traveling in one directionthrough the AWG it may behave as a wavelength division multiplexing(WDM) multiplexer, and for light traveling in the opposite direction, itmay behave as a WDM demultiplexer. Because light may travel in eitherdirection through the AWG, the output waveguides 120 may be used asinputs, and the input waveguides 110 may be used as outputs.Accordingly, each of the input waveguides 110 and the output waveguides120 may be referred to as a “port” waveguide.

The free spectral range (FSR) of an AWG may be related to theincremental delay length (ΔL) of the waveguide array by the expressionFSR=c/(n_(g) ΔL), where n_(g) is the group index of the waveguide usedin the waveguide array and depends on the fabrication platform, and c isthe speed of light in vacuum. This expression is plotted in FIG. 1B inthe case of a 3 um silicon on insulator (SOI) platform. The FSR of anAWG, on the other hand, may be larger or equal to the productN_(Ch)×Ch_(Spac), in order, for example, to have each channel within therange of interest univocally routed out of the corresponding output portof the AWG. A trade-off thus emerges between the productN_(Ch)×Ch_(Spac) and the incremental delay length (ΔL): a smallincremental delay length (ΔL) may be used for an AWG with a large numberof channels or a large channel spacing (or both). In the case of arectangular AWG layout, the minimum incremental delay length (ΔL) may beconstrained by the minimum transverse separation AWG between thewaveguides, which in turn may be constrained to prevent excessive modeoverlap or physical overlapping of the waveguides (FIG. 1C). For thisreason it may not be feasible to achieve more than 16 channels at achannel separation of 100 GHz, with a rectangular AWG layout fabricatedon a 3 um SOI platform.

Referring to FIG. 2A, in a rectangular AWG design, the length differencebetween an innermost waveguide 205 of the array of waveguides and anoutermost waveguide 210 of the array of waveguides may be adjusted bymoving the respective horizontal portions up or down as shown by thearrows, but the minimum length difference that may be achieved isconstrained if the waveguides 205, 210 are to avoid interfering witheach other or with other waveguides that may exist between them.

By contrast, in the T-shaped array of waveguides of the AWG of FIG. 2B,the outermost waveguide 215 may be lengthened, without interfering withother waveguides of the array, by moving one or both of the upperhorizontal portions upward (as shown by two upper arrows 220), and theinnermost waveguide 225 may be lengthened, without interfering withother waveguides of the array, by moving the lower horizontal portiondownward (as shown by the lower arrow 230). As such, the innermostwaveguide 225 may be longer or shorter than the outermost waveguide 215,and the smallest length difference achievable is not affected byconstraints on the minimum transverse separation between adjacentwaveguides. Star couplers 130, 150 are shown schematically in FIGS. 2Aand 2B. Moreover, the layout of the AWG of FIG. 2B facilitates theinclusion of a relatively large number waveguides in the array. Theability to include a relatively large number of waveguides may beadvantageous in AWG designs in which the number of waveguides in thearray is 3-6 times the greater of (i) the number of input channels and(ii) the number of output channels.

Referring to FIG. 3, in some embodiments each waveguide of the array mayinclude, along the waveguide in a direction from the first star couplerto the second coupler, a first straight section 305 (inside an apertureof the first star coupler 130), a first curved section 310, a secondstraight section 315, a first clockwise bend 320, a third straightsection 325, a second clockwise bend 330, a fourth straight section 335,a first counterclockwise bend 340, a fifth straight section 345, asecond counterclockwise bend 350, a sixth straight section 355, a thirdclockwise bend 360, seventh straight section 365, a fourth clockwisebend 370, an eighth straight section 375, a second curved section 380,and a ninth straight section 385 (inside an aperture of the first starcoupler 130).

As such, each waveguide of the array may include four clockwise bendsand two counterclockwise bends, along the waveguide in a direction fromthe first star coupler to the second coupler, or, equivalently, eachwaveguide of the array may include four counterclockwise bends and twoclockwise bends, along the waveguide in a direction from the second starcoupler to the first coupler. In some embodiments some of the straightsections may be absent. For example, the third straight section 325 andthe seventh straight section 365 may be absent for the innermostwaveguide, and/or the fifth straight section 345 may be absent for theoutermost waveguide.

A “bend” or a “curved section” as used herein, is a section of waveguidewithin which the curvature is in one direction, e.g., clockwise whenprogressing along the wavelength in one direction and counterclockwisewhen progressing along the wavelength in the opposite direction.Although in general a bend may be referred to as a curved section, andvice versa, the convention herein is to use the term “bend” to refer tosections of waveguide having a relatively short radius of curvature(e.g., less than 200 microns) and resulting in a significant change indirection (e.g., more than 60 degrees, and to use the term “curvedsection” to refer to sections of waveguide having a relatively longradius of curvature (e.g., between 0.5 mm and 20 mm) and resulting in arelatively small change in direction (e.g., less than 10 degrees).

Bends may be counted according to the total amount of direction change.For example, a sharply curved portion of the waveguide in which thedirection changes by 180 degrees may be referred to as a single 180degree bend, or, equivalently, as two 90 degree bends. Two sharplycurved portions, separated by a straight section, may be referred to astwo 90 degree bends if the direction change in each of them is 90degrees, or they may be referred to as a single 180 degree bend. Eachstraight section may have a curvature of less than 0.01/mm. In someembodiments each bend of each waveguide is substantially identical tothe corresponding bends of all of the other waveguides of the array, sothat phase effects of the bends are common mode and the phasedifferences between the waveguides are due only to length differences.In some embodiments all of the clockwise bends have a first shape, andall of the counterclockwise bends have a second shape. In someembodiments each counterclockwise bend has a shape that is a mirrorimage of the shape of each of the clockwise bends.

The entire structure may be compact, having an overall length L, and anoverall width W, as shown, and occupying an effective chip area of L×W.W may be between 1.5 mm and 14 mm, or, in some embodiments, between 3 mmand 7 mm, and L may be between 4 mm and 28 mm or, in some embodiments,between 8 mm and 14 mm. The effective chip area may be between 6 mm² and35 mm². For example, in one embodiment, an arrayed waveguide gratingwith 24 channels and a channel spacing of 100 GHz has dimensions of 3mm×8 mm. In another embodiment, an arrayed waveguide grating with 48channels and a channel spacing of 100 GHz has dimensions of 7 mm×14 mm.

In some embodiments, each of the waveguides of the waveguide array is arib waveguide along one or more portions of its length. Referring toFIG. 4A, the waveguide may be fabricated as a silicon on insulator (SOI)structure, in which a layer of silicon (Si) 3 microns thick, on a layerof silicon dioxide (SiO₂) (which may be referred to as “buried oxide” or“BOX” layer) is etched to form a slab portion 410 and a rib portion 420extending above the slab portion 410. In one embodiment, the width w_(r)of the rib is 3.0 microns, the height h_(r) of the rib is 1.2 microns,and the height h_(s) of the slab is 1.8 microns. A thin (e.g., 0.2micron thick) layer 430 of silicon may remain on the silicon dioxide inregions on both sides of the slab, for fabrication purposes; this layermay have a negligible effect on the optical characteristics of thewaveguide. Adjacent waveguides in the waveguide array 120 may share aslab portion 410 (as shown in FIG. 4C).

Each waveguide may have a rib cross section in the curved sections 310,380. The curved rib waveguide may shed higher order modes (i.e., confinethem sufficiently poorly that their attenuation within these portions isgreat, e.g., more than 1000 dB/cm), and as a result any light coupledinto the bends 320, 370 adjacent to the curved sections 310, 380 may besubstantially entirely in the fundamental modes.

Within the bends, and within the straight sections 325, 335, 345, 355,365, the waveguides may be strip waveguides, as illustrated in FIG. 4B,including a strip 440 and lacking a slab portion. The strip may have aheight equal to the combined height of slab portion 410 and rib portion420, i.e., a height of h_(s)+h_(r). The strip waveguides may be suitablefor forming tight (<200 micron, or even tighter) bend radii withoutunacceptable optical loss and with minimal coupling from the fundamentalmodes into higher order modes. They may also be multi-mode waveguides.

Tapering, i.e., gradual changes in the cross section along the length ofthe waveguide, may be used to transition between rib and stripwaveguides, and to transition to wide rib cross sections that mayprovide improved coupling to the free propagation regions of the starcouplers 130, 150. Each transitions between rib waveguides stripwaveguides may be referred to as a “rib to strip converter”, having a“rib end” connected to a rib waveguide, and a “strip end” connected to astrip waveguide. Each rib to strip converter may include a region inwhich the slab portion 410 of each of the rib waveguides tapers tobecome progressively narrower until it is the same width as thecorresponding rib portion 420 and is no longer distinct from the ribportion 420. To the extent that higher order modes are suppressed by thecurved portions 310, 380, and that the rib to strip converters do notcouple light into higher order modes, the light coupled into the stripwaveguides of the bends may be entirely in the fundamental modes. Thecoupling of light into the rib to strip converters may be reduced byfabricating the rib to strip converters to be straight (i.e., notcurved) sections of waveguide; for example, the rib to strip convertersmay be formed in the second straight section 315 and the eighth straightsection 375.

FIG. 4C shows a cross section of a portion of the waveguide array on therib end of a rib to strip converter. In the embodiment of FIG. 4C, therib waveguides share a slab portion 410. FIG. 4D shows a cross sectionof a portion of the waveguide array at a point within the rib to stripconverter. A trench 445 that extends nearly to the bottom of the slabportion, half-way between each pair of adjacent ribs, begins at the ribend of the rib to strip converter and then widens in the direction ofthe strip end of the rib to strip converter.

Each waveguide of the waveguide array may have a curvature that isadiabatic along the length of the waveguide, i.e., a rate of change ofcurvature that does not exceed a set value, e.g., a value in a rangefrom 1/mm² to 20/mm², e.g., 5/mm², 10/mm², or 15/mm². As used herein,the “curvature” of the waveguide is the reciprocal of the radius ofcurvature. For example, portions (such as the curved sections 310, 380,and the bends 320, 330, 340, 350, 360, 370) of each waveguide of thewaveguide array may have the shape of a portion of an Euler spiral,which follows a curve for which the rate of change of curvature withdistance along the curve is constant. For example, a curved portion of awaveguide of the waveguide array may have the shape of an Euler arc,which consists of two symmetric portions of an Euler spiral. As usedherein, an “Euler arc” (or “Euler bend”) is symmetric about itsmidpoint, has a curvature that is greatest at its midpoint and vanishesat each of the two ends of the Euler arc, and that changes at a constantrate in each half of the Euler arc, the rate of change of curvaturebeing equal in magnitude, and opposite in sign, in the two halves of theEuler arc. The term “Euler curve” is used herein to refer to anyportion, of an Euler spiral, that has a vanishing curvature at one end.

The absence of discontinuities in the curvature of the waveguide mayprevent coupling into higher order modes that otherwise may occur atsuch a discontinuity. Moreover, as mentioned above, a curved section ofrib waveguide (as, e.g., the curved sections 310, 380) may act as a modefilter, effectively confining only the fundamental (TEO and TMO) modes.

Waveguides fabricated using photolithography or other fabricationtechniques employed to fabricate photonic integrated circuits may havewalls with small-scale (e.g., nm-scale) roughness. This roughness mayresult in each wall of the waveguide having a local curvature, on asmall scale, that is relatively large and fluctuates significantly alongthe length of the waveguide. This local roughness, however, may haverelatively little effect on the propagation of light in the waveguide,and on the coupling between fundamental modes and leaky higher ordermodes. Accordingly, the curvature of a waveguide (as distinct from thelocal curvature of a wall of the waveguide) is defined herein as thecurvature of that would be measured if the small-scale roughness of thewaveguide is disregarded. The curvature of a waveguide may be measured,for example, with an optical microscope, which may be insensitive tofeatures (such as waveguide wall roughness) that are significantlysmaller than the wavelength of visible light.

Although a 5×8 arrayed waveguide grating is illustrated in FIG. 3,having 5 waveguides at the external end of the first star coupler 130and 8 waveguides at the external end of the second star coupler second,other embodiments may be fabricated in an analogous manner to be M×Narrayed waveguide gratings, having M first waveguides and N secondwaveguides, with M and N having integer values that may differ from 5and 8 respectively, and may be as small as 1. Similarly, furtherembodiments may be fabricated to be cyclic N×N arrayed waveguidegratings or non-cyclic N×N arrayed waveguide gratings. Embodiments ofthe invention may be fabricated in any high index contrast systemsuitable for forming tight 90 degree bends, e.g., silicon on insulator(SOI), indium phosphide (InP), or silicon nitride/silicon dioxide(SiN/SiO₂).

FIG. 5 shows a schematic view of a T-shaped arrayed waveguide gratingthat lacks the two outermost bends (e.g., that lacks the first andfourth clockwise bends 320, 370) of the embodiment of FIG. 3, but isotherwise analogous. FIGS. 6A and 6B show a tunable T-shaped arrayedwaveguide grating including a tuning section 610. In the tuning section610, each waveguide of a subset of the waveguides (the subset eitherincluding all of the waveguides, or being a proper subset, andincluding, e.g., all but one of the waveguides) includes a waveguidesection within which the effective index of refraction may be adjusted,e.g., using temperature tuning (using an individual heater on eachwaveguide or a global heater with gradient heat profile) or using aphase modulator in each waveguide of the subset. In this manner, if thelengths of the waveguides are all the same, then when the tuning section610 is adjusted so that all of the waveguide sections have the sameeffective index of refraction (so that the effective lengths are alsoall the same, i.e., the effective length difference is zero for any pairof waveguides), monochromatic light fed into the central input will exitfrom the central output (as shown in FIG. 6A). If the tuning section 610is adjusted so that the waveguide sections do not all have the sameeffective index of refraction (e.g., so that there is a difference ineffective length, that is the same between any pair of adjacentwaveguides), then monochromatic light fed into the central input mayexit from another output (as shown in FIG. 6B). A tunable T-shapedarrayed waveguide grating such as that of FIGS. 6A and 6B may also beused as an arrayed waveguide grating with a tunable, and arbitrarylarge, free spectral range.

An arrayed waveguide grating (or, more generally a multi-pathinterference filter, as discussed in further detail below) may bedesigned so that the optical path delay difference between a firstwaveguide and a second waveguide of the arrayed waveguide grating (i.e.,the difference between (i) the phase delay incurred by propagation alongthe first waveguide and (ii) the phase delay incurred by propagationalong the second waveguide) has a certain design value (e.g., a valuedepending on the desired center wavelength λ_(c), and on the desiredgrating order m (m being an integer) which in turns depends on thedesired FSR of the device). This design criterion may be written:

${( {L_{2} - L_{1}} ) = {m\frac{\lambda_{c}}{n}}},$

which in turns results in a phase difference:

$\begin{matrix}{{{\frac{2\pi \; n}{\lambda_{c}}( {L_{2} - L_{1}} )} = {m\; 2\pi}},} & (1)\end{matrix}$

where L₁ and L₂ are the lengths of the first and second waveguidesrespectively, and n is the effective index of refraction of thewaveguides.

Changes, with temperature, in the index of refraction of an arrayedwaveguide grating may result in changes in the characteristics of thearrayed waveguide grating, e.g., in the center wavelengths of thechannels, and this may, in turn, result in a degradation in systemperformance. Accordingly, in some embodiments the effects of temperaturechanges are reduced, in a device referred to as an athermal arrayedwaveguide grating, by using sections of waveguide having differentthermo-optic coefficients. As used herein, the “thermo-opticcoefficient” of a waveguide is the rate of change of the waveguide'seffective index of refraction with temperature. Referring to FIG. 7,each waveguide of an arrayed waveguide grating may include a firstportion having a first effective index of refraction n₁, and a firstthermo-optic coefficient dn₁/dT, and a second portion having a secondeffective index of refraction n₂, and a second thermo-optic coefficientdn₂/dT. The lengths of the respective first and second portions maydiffer from waveguide to waveguide. For example, a first waveguide 701may have a first portion with a length L₁₁ and a second portion with alength L₂₁, a second waveguide 702 may have a first portion with alength L₁₂ and a second portion with a length L₂₂, and a third waveguide703 may have a first portion with a length L₁₃ and a second portion witha length L₂₃. Equation (1) may then be generalized as follows:

$\begin{matrix}{{\frac{2\pi}{\lambda_{c}}\lbrack {{n_{1}( {L_{12} - L_{11}} )} + {n_{2}( {L_{22} - L_{21}} )}} \rbrack} = {m\; 2{\pi.}}} & (2)\end{matrix}$

If, for any pair of waveguides (e.g., for the pair of waveguidesconsisting of the first waveguide 701 and the second waveguide 702), thefollowing equation is also satisfied:

$\begin{matrix}{{{{\frac{{dn}_{1}}{dT}\Delta \; L_{1}} + {\frac{{dn}_{2}}{dT}\Delta \; L_{2}}} = 0},} & (3)\end{matrix}$

the first order temperature effect (i.e., the rate of change, withtemperature, of the difference between (i) the optical delay through thefirst waveguide 701 and (ii) the optical delay through the secondwaveguide 702) may vanish. In Equation (3), ΔL₁ is the differencebetween the lengths of the respective first portions and ΔL₂ is thedifference between the lengths of the respective second portions. Forexample, for the first waveguide 701 and the second waveguide 702ΔL₁=L₁₂−L₁₁ and ΔL₂=L₂₂−L₂₁. From Equation (3) it may be seen that ifthe thermo-optic coefficients have the same sign

$( {{i.e.},{{{if}\mspace{14mu} \frac{{dn}_{1}}{dT}\frac{{dn}_{2}}{dT}} > 0}} ),$

then ΔL₁ and ΔL₂ have opposite signs (i.e., ΔL₁ΔL₂<0).

If, as may be the case for an arrayed waveguide grating, the differencebetween (i) the total optical delay of the first waveguide 701 and (ii)the total optical delay of the second waveguide 702 is the same as thedifference between (i) the total optical delay of the second waveguide702 and (ii) the total optical delay of the third waveguide 703, thenEquation (3) may be satisfied for the pair of waveguides consisting ofthe second waveguide 702 and the third waveguide 703 for the same valuesof ΔL₁ and ΔL₂, i.e., Equation (3) may be satisfied for this pair ofwaveguides if L₁₃−L₁₂=ΔL₁ and L₂₃−L₂₂=ΔL₂.

In some embodiments, an athermal arrayed waveguide grating may beconstructed using waveguides having portions composed of crystallinesilicon (c-Si) and portions composed of silicon nitride (SiN). Thecrystalline silicon portions may have a thermo-optic coefficient of1.84e-4/° C., and the silicon nitride portions may have a thermo-opticcoefficient of 2.45e-5/° C. FIG. 8 shows an example of a rectangulararrayed waveguide grating design using waveguides composed ofcrystalline silicon, except in a triangular region 805, in which theyare composed of silicon nitride. A design such as that of FIG. 8 mayhave a vanishing first order temperature effect, at an operatingtemperature for which it is designed. Fabricating a device like that ofFIG. 8 may be challenging, however, because of challenges that arise infabricating silicon nitride structures with a thickness suitable forwaveguides, and in integrating such structures with the crystallinesilicon structures of the remainder of the arrayed waveguide grating.Moreover, the relatively large mismatch between the index of refractionof silicon nitride and the index of refraction of crystalline siliconmay limit the performance of an arrayed waveguide grating fabricatedfrom these materials.

In other embodiments, an athermal arrayed waveguide grating may insteadbe constructed using waveguides having portions composed of crystallinesilicon and portions composed of hydrogenated amorphous silicon(a-Si:H). The hydrogenated amorphous silicon portions may have athermo-optic coefficient of 2.3e-4/° C. The relatively small differencebetween the thermo-optic coefficient of crystalline silicon and thethermo-optic coefficient of hydrogenated amorphous silicon may be anobstacle to constructing a rectangular athermal arrayed waveguidegrating from these materials, but a T-shaped arrayed waveguide grating,such as that shown in FIG. 9, or a horseshoe shaped arrayed waveguidegrating, such as that shown in FIG. 10, may be fabricated from thesematerials. In each of the embodiments of FIGS. 9 and 10, the waveguidesof the arrayed waveguide grating are composed of crystalline siliconexcept in respective triangular regions 905, 1005, in which they arecomposed of hydrogenated amorphous silicon. These devices may befabricated, from a wafer having a crystalline silicon upper surface, byfirst replacing the crystalline silicon in a triangular region withhydrogenated amorphous silicon, and then masking and etching thewaveguides and star couplers of the arrayed waveguide grating structurein subsequent steps.

For example, in some embodiments, a mask (e.g., an oxide hard mask)having an aperture of a suitable shape and size (e.g., a triangularaperture) is formed on the upper layer of silicon of a silicon oninsulator (SOI) wafer. A corresponding (e.g., triangular) cavity is thenetched into the top surface of the SOI wafer using a suitable etchingprocess (e.g., a reactive ion etching process or another inductivelycoupled plasma process). This etch may remove, in the region of the maskaperture, the upper layer of silicon of the SOI wafer (down to theburied oxide (BOX) layer of the SOI wafer). In some embodiments, anothermask, having an aperture slightly larger than the aperture of the oxidehard mask, may then be formed on the SOI wafer, and hydrogenatedamorphous silicon may be deposited on the SOI wafer. The mask may thenbe removed, leaving a layer of hydrogenated amorphous silicon fillingthe cavity and overlapping onto the oxide hard mask in a strip aroundthe perimeter of the cavity. In other embodiments, one of variousdifferent processes may be employed, For example, after the cavity isformed, hydrogenated amorphous silicon may be deposited over the entirewafer, and CMP may then be performed to remove it except from within thecavity. The hydrogenated amorphous silicon may be deposited using, forexample, plasma-enhanced chemical vapor deposition (PECVD) using SiH₄ ata flow rate of 70 standard cubic centimeters per minute (sccm), argon at210 sccm, a substrate temperature of 300° C., a pressure of 500 mTorr,radio frequency (RF) power of 250 W, an RF frequency of 380 kHz, and adeposition rate of 96 nm per minute.

The characteristics of the deposited hydrogenated amorphous silicon mayvary, depending on the process and process parameters used to depositit. In some embodiments, a variant of the process described above may beused, in which a different value is used for one or more of theparameters listed (e.g., a different flow rate, a different temperature,or a different RF power or frequency). In some embodiments RF power attwo or more frequencies is applied during the PECVD process (and thepower at all frequencies may not be the same). The process may beadjusted empirically to achieve one or more of several objectives forthe characteristics of the deposited hydrogenated amorphous silicon,including (i) low stress (e.g., less than +/−50 MPa) (to avoid stressinduced birefringence and unacceptable bowing of the wafer), (ii) anindex of refraction similar to that of (e.g., within 10% of that of)crystalline silicon (to avoid high loss at interfaces), (iii) as largeas possible a thermo-optic coefficient (i.e., differing as much aspossible from that of crystalline silicon, making possible the design ofsmall devices), (iv) low optical loss, and (v) ability to withstand hightemperatures in subsequent processing steps, without unacceptablehydrogen out-diffusion. In some embodiments, once a process has beenselected (and process parameters have been selected) for depositing thehydrogenated amorphous silicon, the optical characteristics (e.g., theindex of refraction, the thermo-optic coefficient, and higher-ordercharacteristics such as the rate of change of the thermo-opticcoefficient with temperature or with wavelength) may be measured from atest film deposited using the process, and the dimensions of portions ofhydrogenated amorphous silicon in the device to be fabricated may thenbe selected so that acceptable temperature compensation is achieved inthe device. This approach may be employed, provided the process fordepositing the hydrogenated amorphous silicon produces hydrogenatedamorphous silicon with repeatable characteristics, even if thedependence of the characteristics of the hydrogenated amorphous siliconon the process parameters is not fully understood.

In some embodiments the shape of the hard mask is chosen to produceangled interfaces between the waveguide portions composed ofhydrogenated amorphous silicon and the waveguide portions composed ofcrystalline silicon, to reduce the effect of reflections (by avoidingthe production of reflections that are mode-matched to the waveguides).In some embodiments each interface between a waveguide portion composedof hydrogenated amorphous silicon and a waveguide portion composed ofcrystalline silicon is a substantially planar surface having a surfacenormal that is at an angle greater than or equal to 0 degrees and lessthan 30 degrees with respect to the longitudinal direction of one orboth of the two waveguide portions. The angles of the interfaces betweenthe waveguide portions composed of hydrogenated amorphous silicon andthe waveguide portions composed of crystalline silicon may be aconsequence of slopes of the edges of the aperture in the oxide hardmask (e.g., each interface may be parallel to an edge of the triangle,if a mask with a simple triangular aperture is used), or the maskaperture may have edges each including a plurality of straight segments,a segment at each point where a waveguide will be formed having adirection corresponding to a design interface angle.

A total thickness of hydrogenated amorphous silicon exceeding thethickness of the upper layer of silicon of the SOI wafer may bedeposited (e.g., 5 microns of hydrogenated amorphous silicon may bedeposited, on an SOI wafer for which the upper layer of silicon has athickness of 3 microns). Chemical mechanical polishing (CMP) may then beused to remove the excess hydrogenated amorphous silicon, so that theremaining hydrogenated amorphous silicon just fills the cavity, and sothat the upper surface of the hydrogenated amorphous silicon is flushwith the upper surface of the surrounding oxide hard mask. The oxidehard mask may also be used as an etch stop for the CMP process. Theoxide hard mask may be removed in a subsequent processing step.

Waveguides may then be etched in two steps (which may be performed ineither order): (i) in one step the region of hydrogenated amorphoussilicon may be masked off and waveguides (and, e.g., star couplers) maybe formed by suitable etching of the upper layer of silicon of the SOIwafer, and (ii) in another step, the upper layer of silicon of the SOIwafer may be masked off and waveguides (or other structures) may beformed by suitable etching of the hydrogenated amorphous silicon. Insome embodiments, a self aligned process is used, in which an oxide hardmask is patterned to define continuous crystalline silicon andhydrogenated amorphous silicon waveguides (i.e., continuous waveguideshaving portions of each material), and the hard mask is then furthercovered in resist during two separate subsequent waveguide etching stepsfor crystalline silicon and hydrogenated amorphous silicon respectively.In some embodiments the etch process used for these two steps is thesame (e.g., an inductively coupled plasma process), and the steps areperformed separately because the etch rate achieved by the process isdifferent for hydrogenated amorphous silicon than it is for crystallinesilicon. Any registration error between the masks used for the two etchsteps may result in some degradation in the performance of the resultingstructure, but the degradation may be acceptable or insignificant if theregistration errors are small (e.g., about +/−200 nm or less).Subsequent processing steps involving high temperatures (e.g.,temperatures exceeding 350° C.) may be avoided, to avoid an increase inoptical propagation loss (e.g., due to hydrogen out-diffusion) withinthe hydrogenated amorphous silicon portions of the structure.

The dimensions of the triangular regions 905, 1005 may be selected,using Equations (2) and (3), to cause the first order temperature effectto vanish. Such an approach may be appropriate when the operatingtemperature range is sufficiently small that higher order effects mayremain small. In some embodiments the shape of the region within whichthe waveguides are composed of a different material is not an isoscelestriangle as shown, but is instead a triangle that lacks the symmetry ofan isosceles triangle, or a shape that is not a triangle (e.g., one inwhich one or more of the sides of the triangle 905 have been replacedwith curved lines) while preserving propagation lengths within thedifferent materials that satisfy Equation (2), and that satisfy Equation(3) (or a similar requirement for applications in which higher ordereffects are significant, as discussed in further detail below). In someembodiments, one waveguide of an array has no hydrogenated amorphoussilicon portion; in other embodiments, one waveguide has a shorthydrogenated amorphous silicon portion which is included so that eachwaveguide of the waveguide array includes two interfaces betweencrystalline silicon portions and hydrogenated amorphous siliconportions, so that losses produced by these interfaces are largely thesame in all of the waveguides. Such balancing of losses may be ofgreater importance in other devices (e.g., in a generalized Mach-Zehnderinterferometer, discussed below) than in an arrayed waveguide grating.

In some embodiments, if the operating temperature range is sufficientlylarge that higher order effects (e.g., second order effects, such as achange in the thermo-optic coefficient with temperature) aresignificant, another figure of merit may be used (instead of designingfor a sufficiently small or vanishing first order temperature effect) todesign the dimensions (and shape, if it is permitted to deviate from atriangular shape) of the region within which the waveguides are composedof a material (e.g. hydrogenated amorphous silicon) other thancrystalline silicon. For example, the maximum change in the centerwavelength of any channel over the operating temperature range may beused as a figure of merit. Simulations show that for the embodiment ofFIG. 9, which has a channel spacing of about 0.8 nm, the maximum changein the center wavelength of any channel over the temperature rangeextending from 20° C. to 70° C. may be as little as 250 pm, whereas fora similar structure in which the waveguides of the arrayed waveguidegrating are composed entirely of crystalline silicon, the maximum changein the center wavelength may be 4 nm.

It may be advantageous for an arrayed waveguide grating to havepolarization-independent characteristics. For the embodiment of FIG. 9,the portions of each waveguide which are strip waveguides withsubstantially square cross sections (which include the portions composedof hydrogenated amorphous silicon) may inherently have lowbirefringence. The portions of each waveguide which are rib waveguidesmay have significant birefringence, but the effect of the birefringencemay (i) be the same in each waveguide of the array, if the rib waveguideportions have the same lengths and shapes (and the behavior of thedevice may therefore be polarization independent) or (ii) be compensatedwhen a layer of thermal oxide 1010 (FIG. 10) is formed on top of asilicon rib waveguide structure, as it induces a physical stress thataffects the relative transmission of the TM and TE polarizations in anopposite way to the overall effect of the sources of birefringenceinherent in the silicon rib waveguide.

Referring to FIGS. 11A and 11B, in some embodiments, an athermal arrayedwaveguide grating may be constructed using one or more areas 1110 ofhydrogenated amorphous silicon, in one or both of the free propagationregions of the star couplers. Each such area 1110 may have awedge-shaped portion within the free propagation region (the area 1110may be wedge-shaped as shown, or, for example, triangular) so that theoptical paths, through two different waveguides of the waveguide array,from a port waveguide of one of the star couplers to a port waveguide ofthe other star coupler, include different lengths within the area 1110.Such areas 1110 of hydrogenated amorphous silicon may be used insteadof, or in addition to, hydrogenated amorphous silicon portions in theotherwise crystalline silicon waveguides. The dimensions of the area orareas may be determined from Equation (3) above, with the quantities ΔL₁and ΔL₂ defined to include paths within the free propagation region (orregions) of the star coupler (or star couplers) containing such areas1110.

Embodiments of the present invention may be used not only in arrayedwaveguide gratings but also in any other optical device, such as amulti-path interference filter, in which differential delays may beaffected by temperature changes, and in which controlling the effects ofsuch temperature effects may be advantageous. As used herein, a“multi-path interference filter” is any optical device which selectswavelengths by dividing one input optical field received at a first portwaveguide (e.g., an input waveguide) into two or more intermediateoptical paths having different optical path lengths and by recombiningthe intermediate optical paths into an output field at a second portwaveguide (e.g., an output waveguide) that is modified by interferencebetween the intermediate paths. The first port waveguide may be followedby a first optical power splitter, which divides the input stream intointermediate optical paths which are recombined by a second opticalpower splitter (operating as a combiner), followed by the second portwaveguide; as such, the optical device includes at least two differentoptical paths from the first port waveguide to the second portwaveguide. Examples of multi-path interference filters includeMach-Zehnder filters. Such a filter may include a splitter, that splitsinput light to a plurality of waveguides, and a combiner that recombinesthe light after portions of the light have propagated through respectiveones of the waveguides. The attenuation and relative delay of thewaveguides may be selected for a desired filter response. The waveguidesmay be athermalized according to embodiments of the present invention,e.g., by including in each waveguide one or more portions composed ofcrystalline silicon and one or more portions composed of hydrogenatedamorphous silicon. In such an athermalized device, the rate of changewith temperature of the optical path delay difference between twodifferent paths between the first port waveguide and the second portwaveguide, at a wavelength of 1550 nm, may be less than 2e-4 radians/°C., e.g., it may be less than 4e-5 radians/° C. or less than 4e-6radians/° C., or less. In some embodiments the maximum change, in theoptical path delay difference, at a wavelength of 1550 nm, over thetemperature range extending from 20° C. to 70° C., is less than 1e-2radians, e.g., it may be less than 2e-3 radians, or less than 2e-4radians, or less.

For example, referring to FIG. 12A, a Mach-Zehnder interferometer mayinclude a first coupler (e.g., a multi-mode interference (MIMI) coupler)1205, a second coupler (e.g., an MMI coupler) 1210, and two waveguides,each of the waveguides having one or more portions 1220 composed ofhydrogenated amorphous silicon and one or more portions 1225 composed ofcrystalline silicon. In the schematic drawing of FIG. 12A, the totallengths of the two waveguides are illustrated to be equal, although insome embodiments they are unequal. The lengths of portions 1220 composedof hydrogenated amorphous silicon and the lengths of portions 1225composed of crystalline silicon may be selected to satisfy Equation (2)and to satisfy Equation (3) (or a similar requirement for applicationsin which higher order effects are significant).

Referring to FIG. 12B, a lattice filter may be formed by concatenating aplurality of Mach-Zehnder interferometers, as shown, with each pairadjacent Mach-Zehnder interferometers sharing a coupler. In theembodiment of FIG. 12B, the lattice filter includes two Mach-Zehnderinterferometers including three couplers (a first coupler (e.g., an MMIcoupler) 1205, a second coupler (e.g., an MIMI coupler) 1210, and athird coupler (e.g., an MIMI coupler) 1215), and two pairs of waveguidesas shown, each of the waveguides having one or more portions 1220composed of hydrogenated amorphous silicon and one or more portions 1225composed of crystalline silicon. In some embodiments an otherwiseanalogous lattice filter includes more than two concatenatedMach-Zehnder interferometers. In the schematic drawing of FIG. 12B, thetotal lengths of the two waveguides of each pair of waveguides areillustrated to be equal, although in some embodiments they are unequal.As is the case for the embodiment of FIG. 12A, the lengths of portions1220 composed of hydrogenated amorphous silicon and the lengths ofportions 1225 composed of crystalline silicon may be selected to satisfyEquation (2) and to satisfy Equation (3) (or a similar requirement forapplications in which higher order effects are significant).

Referring to FIG. 12C, a generalized Mach-Zehnder interferometer mayinclude a first coupler 1235 and a second coupler 1240, the firstcoupler 1235 and the second coupler 1240 being connected by an array ofwaveguides, each of the waveguides having one or more portions 1220composed of hydrogenated amorphous silicon and one or more portions 1225composed of crystalline silicon. In the schematic drawing of FIG. 12C,the respective total lengths of all of the waveguides of the array ofwaveguides are illustrated to be equal, although in some embodimentsthey are unequal. As is the case for the embodiment of FIG. 12A, thelengths of portions 1220 composed of hydrogenated amorphous silicon andthe lengths of portions 1225 composed of crystalline silicon may beselected to satisfy Equation (2) and to satisfy Equation (3) (or asimilar requirement for applications in which higher order effects aresignificant).

As used herein, a “coupler” is a passive linear time-invariant opticaldevice having one or more first ports (e.g., input ports) and one ormore second ports (e.g., output ports) the total number of input andoutput ports being at least three. Examples of couplers include MMIcouplers, star couplers, directional couplers, Y-junctions, andadiabatic splitters.

Referring to FIG. 13A, an echelle grating may be formed with an inputwaveguide 1305, and with a plurality of output waveguides 1310, each ofwhich is a waveguide ending at a boundary with a slab region 1315, theboundary being on a Rowland circle. The grating may be formed along agrating curve 1330 which may be a portion of a circle having twice theradius of the Rowland circle and being tangent to the Rowland circlewithin a region of the grating curve illuminated by light from an inputwaveguide. The grating 1325 may include a series of reflective facets1335 as illustrated in the enlarged view of FIG. 13B. The slab region1315 may include a region 1350 of hydrogenated amorphous silicon asshown. The region of hydrogenated amorphous silicon may have any shapethat results in different optical paths between a first port waveguide(e.g., an input waveguide) and a second port waveguide (e.g., an outputwaveguide) satisfying Equation (2) and satisfying Equation (3) (or asimilar requirement for applications in which higher order effects aresignificant).

Although exemplary embodiments of a T-shaped arrayed waveguide gratinghave been specifically described and illustrated herein, manymodifications and variations will be apparent to those skilled in theart. Accordingly, it is to be understood that a T-shaped arrayedwaveguide grating constructed according to principles of this inventionmay be embodied other than as specifically described herein. Theinvention is also defined in the following claims, and equivalentsthereof.

What is claimed is:
 1. An arrayed waveguide grating, comprising: a firststar coupler; a second star coupler; an array of waveguides connectingthe first star coupler and the second star coupler; one or more firstport waveguides connected to the first star coupler; and one or moresecond port waveguides connected to the second star coupler, wherein: afirst optical path, from a first waveguide of the first port waveguides,through a first waveguide of the array of waveguides, to a firstwaveguide of the second port waveguides, includes a portion, having afirst length, within hydrogenated amorphous silicon, the remainder ofthe first optical path is within crystalline silicon, a second opticalpath, from the first waveguide of the first port waveguides, through asecond waveguide of the array of waveguides, to the first waveguide ofthe second port waveguides, includes a portion, having a second length,within hydrogenated amorphous silicon, the remainder of the secondoptical path is within crystalline silicon, and the second length isdifferent from the first length.
 2. The arrayed waveguide grating ofclaim 1, wherein a rate of change, with temperature, of a centerwavelength of a channel of the arrayed waveguide grating is less than 70pm/° C.
 3. The arrayed waveguide grating of claim 1, wherein: the firstwaveguide of the array of waveguides includes a first portion, having alength equal to the first length, composed of hydrogenated amorphoussilicon, the remainder of the first waveguide of the array of waveguidesis composed of crystalline silicon, the second waveguide of the array ofwaveguides includes a portion, having a length equal to the secondlength, composed of hydrogenated amorphous silicon, and the remainder ofthe second waveguide of the array of waveguides is composed ofcrystalline silicon.
 4. The arrayed waveguide grating of claim 3,wherein an interface between the first portion of the first waveguideand a portion of the remainder of the first waveguide is a substantiallyplanar surface having a surface normal, an angle between the surfacenormal and a longitudinal direction of the first portion being greaterthan 0.1 degrees.
 5. The arrayed waveguide grating of claim 4, whereinthe angle between the surface normal and the longitudinal direction ofthe first portion is less than 30 degrees.
 6. The arrayed waveguidegrating of claim 1, wherein the first star coupler includes a freepropagation region including an area composed of hydrogenated amorphoussilicon, the remainder of the free propagation region of the first starcoupler being composed of crystalline silicon, the area including awedge-shaped portion.
 7. A multi-path interference filter, comprising: afirst port waveguide; a second port waveguide; and an optical structureconnecting the first port waveguide and the second port waveguide, theoptical structure having: a first optical path from the first portwaveguide to the second port waveguide, and a second optical path,different from the first optical path, from the first port waveguide tothe second port waveguide, the first optical path having a portion,having a first length, within hydrogenated amorphous silicon, the secondoptical path having a portion, having a second length, withincrystalline silicon, and the second optical path having either noportion within hydrogenated amorphous silicon, or a portion, having athird length, within hydrogenated amorphous silicon, the third lengthbeing less than the first length.
 8. The multi-path interference filterof claim 7, wherein an optical path delay difference between the firstoptical path and the second optical path has a rate of change, withtemperature, of less than 2e-5 radians/° C.
 9. The multi-pathinterference filter of claim 7, wherein the optical structure comprisesa Mach-Zehnder interferometer having: a first coupler, a second coupler,a first waveguide connecting the first coupler and the second coupler,and a second waveguide connecting the first coupler and the secondcoupler, wherein: a portion of the first optical path is within thefirst waveguide, and a portion of the second optical path is within thesecond waveguide.
 10. The multi-path interference filter of claim 7,wherein the optical structure comprises a generalized Mach-Zehnderinterferometer having: a first coupler, a second coupler, a firstwaveguide connecting the first coupler and the second coupler, a secondwaveguide connecting the first coupler and the second coupler, a thirdwaveguide connecting the first coupler and the second coupler, andwherein: a portion of the first optical path is within the firstwaveguide, and a portion of the second optical path is within the secondwaveguide.
 11. The multi-path interference filter of claim 7, whereinthe optical structure comprises two concatenated Mach-Zehnderinterferometers.
 12. The multi-path interference filter of claim 7,wherein the optical structure comprises an echelle grating.
 13. Themulti-path interference filter of claim 7, wherein an optical path delaydifference between the first optical path and the second optical pathexhibits a maximum change, over a temperature range extending from 20°C. to 70° C., of less than 2e-3 radians.